Antigen-specific activation and proliferation of lymphocytes are regulated by both positive and negative signals from costimulatory molecules. The most extensively characterized T cell costimulatory pathway is B7-CD28, in which B7-1 (CD80) and B7-2 (CD86) each can engage the stimulatory CD28 receptor and the inhibitory CTLA-4 (CD152) receptor. In conjunction with signaling through the T cell receptor, CD28 ligation increases antigen-specific proliferation of T cells, enhances production of cytokines, stimulates differentiation and effector function, and promotes survival of T cells (Lenshow, et al., Annu. Rev. Immunol., 14:233-258 (1996); Chambers and Allison, Curr. Opin. Immunol, 9:396-404 (1997); and Rathmell and Thompson, Annu. Rev. Immunol., 17:781-828 (1999)). In contrast, signaling through CTLA-4 is thought to deliver a negative signal that inhibits T cell proliferation, IL-2 production, and cell cycle progression (Krummel and Allison, J. Exp. Med, 183:2533-2540 (1996); and Walunas, et al., J. Exp. Med., 183:2541-2550 (1996)). Other members of the B7 family include B7-H1 (Dong, et al., Nature Med., 5:1365-1369 (1999); and Freeman, et al., J. Exp. Med., 192:1-9 (2000)), B7-DC (Tseng, et al., J. Exp. Med., 193:839-846 (2001); and Latchman, et al., Nature Immunol., 2:261-268 (2001)), B7-H2 (Wang, et al., Blood, 96:2808-2813 (2000); Swallow, et al., Immunity, 11:423-432 (1999); and Yoshinaga, et al., Nature, 402:827-832 (1999)), B7-H3 (Chapoval, et al., Nature Immunol., 2:269-274 (2001)) and B7-H4 (Choi, et al., J. Immunol., 171:4650-4654 (2003); Sica, et al., Immunity, 18:849-861 (2003); Prasad, et al., Immunity, 18:863-873 (2003); and Zang, et al., Proc. Natl. Acad. Sci. U.S.A., 100:10388-10392 (2003)). B7-H1 and B7-DC are ligands for PD-1, B7-H2 is a ligand for ICOS, and B7-H3 remains at this time an orphan ligand (Dong, et al., Immunol Res., 28:39-48 (2003)).
B7 family molecules are expressed on the cell surface as homodimers with a membrane proximal constant IgC domain and a membrane distal IgV domain. Receptors for these ligands share a common extracellular IgV-like domain. Interactions of receptor-ligand pairs are mediated predominantly through residues in the IgV domains of the ligands and receptors (Schwartz, et al., Nature Immunol., 3:427-434 (2002)). In general, IgV domains are described as having two sheets that each contain a layer of β-strands (Williams and Barclay, Annu. Rev. Immunol, 6:381-405 (1988)). The front and back sheets of CTLA-4 contain strands A′GFC′C and ABEDC,″ respectively (Ostrov, et al., Science, 290:816-819 (2000)), whereas the front and back sheets of the B7 IgV domains are composed of strands AGFCC′C″ and BED, respectively (Schwartz, et al., Nature, 410:604-608 (2001); Stamper, et al., Nature, 410:608-611 (2001); and Ikemizu, et al., Immunity, 12:51-60 (2000)). Crystallographic analysis revealed that the CTLA-4/B7 binding interface is dominated by the interaction of the CDR3-analogous loop from CTLA-4, composed of a MYPPPY motif, with a surface on B7 formed predominately by the G, F, C, C′ and C″ strands (Schwartz, et al., (2001) supra; and Stamper, et al., (2001) supra.). Data from ammo acid homologies, mutation, and computer modeling provide support for the concept that this motif also is a major B7-binding site for CD28 (Bajorath, et al., J. Mol. Graph. Model., 15:135-139 (1997)). Although the MYPPPY motif is not conserved in ICOS, studies have indicated that a related motif having the sequence FDPPPF and located at the analogous position is a major determinant for binding of ICOS to B7-H2 (Wand, et al., J. Exp. Med., 195:1033-1041 (2002)).
B7-DC (also called PD-L2) is a relatively new member of the B7 family, and has an amino acid sequence that is about 34% identical to B7-H1 (also called PD-L1). Human and mouse B7-DC orthologues share about 70% amino acid identity. While B7-H1 and B7-DC transcripts are found in various tissues (Dong, et al. (1999) supra; Latchman, et al. (2001) supra; and Tamura, Blood, 97:1809-1816 (2001)), the expression profiles of the proteins are quite distinct. Expression of B7-H1 protein, although essentially not found in normal tissues other than macrophage-like cells, can be induced in a variety of tissues and cell types (Dong, et al. (1999) supra; Tamura, et al. (2001) supra; and Ishida, et al., Immunol. Lett., 84:57-62 (2000)). In contrast, B7-DC is expressed only in dendritic cells and monocytes (Tseng, et al. (2001) supra; and Ishida, et al. (2000) supra).
It has been shown that both B7-H1 and B7-DC bind to PD-1 (programmed cell death-1) (Freeman, et al., J. Exp. Med., 192:1027-1034 (2000); Tseng (2001) supra; Latchman (2001) supra), a distant member of the CD28 family with an immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytoplasmic domain (Ishida, et al., EMBO J., 11:3887-3895 (1992)). PD-1 is expressed on a subset of thymocytes and up-regulated on T, B, and myeloid cells after activation (Agata, et al., Int. Immunol., 8:765-772 (1996)). The phenotypes of PD-1−/− mice provide direct evidence for PD-1 being a negative regulator of immune responses in vivo. In the absence of PD-1, mice on the C57BL/6 background slowly develop a lupus-like glomerulonephritis and progressive arthritis (Nishimura, et al., Immunity, 11:141-151 (1999)). PD-1−/− mice on the BALB/c background rapidly develop a fatal autoimmune dilated cardiomyopathy (Nishimura, et al., Science. 291:319-322 (2001)). However, substantial evidence indicates that B7-DC can function to costimulate T cell responses. In the presence of suboptimal TCR signals, B7-DC stimulates increased proliferation and production of cytokines in vitro (Tseng, et al., J. Exp. Med. 193:839-846 (2001)). On the other hand, in vitro studies indicate a negative regulatory role for B7-DC in T cell responses (Latchman (2001) supra). These seemingly contradictory data are best interpreted by expression of additional receptors for B7-DC on T cells other than PD-1.
It would be advantageous to provide compositions that increase antigen-specific proliferation of T cells, enhance production of cytokines, stimulate differentiation and effector function, and promote survival of T cells. It would also be advantageous to provide B7-DC variant polypeptides that have reduced binding affinity for PD-1 compared to wild type B7-DC, yet retain the ability to costimulate T cells (i.e., increase antigen-specific proliferation of T cells, enhance cytokine production by T cells, stimulate differentiation and effector functions of T cells, or promote survival of T cells).
It is therefore an object of the present invention to provide B7-DC variant polypeptides that have reduced binding affinity for PD-1 compared to wild type B7-DC, yet retain the ability to costimulate T cells.
It is another object of the present invention to provide isolated nucleic acid molecules encoding variant B7-DC polypeptides.
It is another object of the present invention to provide cells containing vectors that express nucleic acid molecules encoding variant B7-DC polypeptides.
It is a still further an object of the present invention to provide methods for costimulating T cells by contacting them with variant B7-DC polypeptides.
It is still a further object of the invention to provide methods for administering variant B7-DC polypeptides, nucleic acids encoding the same, or cells transfected or transduced with nucleic acids encoding variant B7-DC polypeptides to a mammal in need thereof.
It is still a further object of the invention to provide methods for potentiating an immune response to an antigen or a vaccine by administering variant B7-DC polypeptides in combination with the antigen or vaccine.